This invention relates to power MOSFETs, and more specifically relates to a novel structure for a power integrated circuit wherein both the drain and source regions can be placed at high voltage relative to the chip substrate to enable its use as a high-side switch.
Power integrated circuits are known in which a power MOSFET and signal processing circuit for controlling the power MOSFET are integrated into the same chip. Such power integrated circuits are used in low-side switching applications. By "low-side switching" is meant an application in which the MOSFET source is held to about the same potential as the chip substrate while the drain can be at relatively high potential relative to the substrate, typically 100 volts or more. The integrated control circuit typically may be a CMOS-type circuit operated at low voltage, for example 15 volts or less relative to the substrate. This control circuit is easily integrated into the substrate which is at ground potential.
There are many applications for power semiconductor switching devices which put the semiconductor device on the high-side. In those circuit applications, the semiconductor switch is connected between the high voltage input potential and the load. Consequently, both the source and drain electrodes of the power MOSFET component will be placed at high or line potential. In applications where the line potential is above 200 volts, it has been impossible to integrate a grounded control circuit into the same substrate since the substrate can be only 100 volts or less from the source. If the source is greater than about 100 volts higher than the substrate, avalanche, punch through breakdown, or pinch-off would occur when the MOSFET device is turned on. The integrated circuit components, however, are conventionally low voltage MOSFET parts which must be near ground potential to communicate with the rest of the circuit which is off the chip. Consequently, discrete power MOSFETs with separate, off the chip controls are used in high-side switching applications.